This invention relates to an angular position detector for determining the position of a shaft or other rotary member. It relates especially to an optical angular position detector which uses a plurality of light emitters and light detectors to achieve that determination and to a detection method utilizing same.
An angular position detector is often used in conjunction with an optical scanner to cause a light beam, e.g. a laser beam, to follow a selected path. The scanner may include a motor which rotates or oscillates a shaft supporting a mirror which intercepts the beam and redirects it to follow a selected path on a surface in order to perform work of some kind, e.g. etching, heating, scribing, photoexposing, etc. For that beam path to be true, it is essential that the position detector provide an accurate real-time indication of the current shaft position at all operative angles of the shaft.
Many present day scanners have position detectors which utilize a moving-dielectric-element capacitance sensor to monitor the shaft position because that type detector is highly accurate. In other words, the output of the detector is substantially linear, i.e. 99.9%, at all operative positions of the shaft, e.g. out to ±25°. However, capacitive-type position detectors do have drawbacks which limit their use for some applications. More particularly, they require an oscillator to excite the capacitance sensor. This increases the overall size and cost of the position detector. Also, capacitive position detectors are susceptible to interference from RF sources such as cell phones, and the output of such detectors can drift with changes in temperature and/or humidity.
To avoid such problems, optical angular position detectors are being used in many present day scanners. Conventional such detectors comprise a plurality of light detectors arranged circumferentially around a center point or axis and a light blocker spaced in front of and parallel to the detectors and arranged to be rotated about said axis. Typically, the blocker is mounted to the end of a rotary scanner shaft extending along said axis. The detectors are arranged to receive light from a light source, e.g. a LED, that is not intercepted by the blocker. The light source may be a single emitter located in front of the blocker so that it can illuminate all exposed areas of all the detectors; see U.S. Pat. No. 5,844,673. Alternatively, as shown in U.S. Pat. No. 6,921,893, a plurality of light emitters may be located beside the detectors with the light from those emitters being directed to a reflector spaced in front of the blocker so that diffuse light from all the emitters is reflected back to all areas of all the detectors that are not covered by the blocker. In other words, from the point of view of the detectors the light from all the emitters is circumferentially uniform.
Thus, in both cases, the blocker exposes all of the detectors to light from all of the emitters in an amount which corresponds to the angular position of the blocker about said axis. Resultantly, the position detector produces an electrical output which may be correlated to the angular position of the shaft. That output may then be processed by a control circuit to provide a determination of the shaft position.
One major problem with the existing optical position detectors is that the shaft position accuracy is not as high as that of a conventional capacitive position detector. In other words, the output of the optical position detector is not actually linear for all angular excursions of the shaft. Whereas a capacitive position detector may have an accuracy of 99.9% out to ±20° shaft angle as noted above, the output of known optical position sensors may only be linear out to the standard scanning angle of ±10°, with the linearity falling off by as much 0.5-1.0% for angles greater than that. Accordingly, it would be desirable to be able to provide an optical position detector which has the positional accuracy of a capacitive detector while avoiding the above-mentioned drawbacks of the latter.